Should be S or arrow key down, but apparently there's a bug for some people where the controls don't work. It's on my list of bugs to fix. It seems that sometimes the issue is USB devices like a HOTAS interfering with the inputs.
ground pound = smashing/pounding the ground with your legs like what mario does when you jump and press down button... It definitely does not have any deep meaning like screwing your friend's mom in her you know what.
